Filters with stringent amplitude and group delay requirements over a passband region are widely used in modern communications systems.
FIG. 1a shows a top view of a conventional nine element linear phase filter 10 comprised of a housing 11 having rod-shaped filter elements 12, 13, 14, 15, 16, 17, 18, 19, and 20, and metal walls 21, 22, 23, 24 and 25. Filter 10 has three cross couplings, namely between the pairs of elements 13 and 19, 14 and 18, and 15 and 17. Walls 23, 24 and 25 form a gap 26 to allow the cross coupling between filter elements 15 and 17, by allowing the coupling between the pairs of the elements 15 and 16 and 16 and 17.
In symmetrical filters, the first element to filter a signal, e.g. element 12, and the last element to filter the signal, e.g., element 20, are identical and the spacing between each of these respective elements and the elements adjacent to them are also identical. Similarly, in symmetrical filters the second and next to last filter elements. e.g., elements 13 and 19, respectively, are identical, the third and second to last filter elements, e.g., elements 14 and 18, respectively, are identical, and so on. Symmetrical filters are easier and thus less expensive to fabricate, align and tune than non-symmetrical filters since each filter half in the symmetrical filter is identical.
In filters of the type shown in FIG. 1a, energy is transferred between coupled filter elements, e.g., between physically adjacent elements. For example, in filter 10 shown in FIG. 1a, element 15 is coupled to element 16 which is coupled to element 17. A pair of filter elements are cross-coupled when each element is coupled to the other in addition to at least two other filter elements. For example, in filter 10 shown in FIG. 1a, element 15 is cross-coupled to element 17 because elements 15 and 17 are coupled to each other and coupled to at least two additional filter elements, for example element 15 is coupled to elements 14, 16 and 17 and element 17 is coupled to elements 15, 16 and 18. Filters having cross-coupled elements have better operating characteristics than filters having only serially coupled elements. Specifically, whereas filters having only serially coupled elements can attain either a desired amplitude flatness or a desired gain flatness but not both, filters having cross-coupled elements can attain both a desired amplitude flatness and a desired gain flatness.
FIG. 1b shows a top view of conventional non-symmetrical nine element linear phase filter 27 comprised of a housing 28 having rod-shaped filter elements 29, 30, 31, 32, 33, 34, 35, 36 and 37, and metal walls 38 and 39. A single cross coupling is produced between the element pair 32 and 35. As mentioned above, non-symmetrical filters are more difficult to fabricate and tune than symmetrical filters. Filter 27 has a single cross coupling which is easier to implement than a filter having multiple cross couplings. However, the single cross coupling does not overcome the aforementioned drawbacks of non-symmetrical filters. Filters having a single cross coupling are easier and thus less expensive to fabricate, align and tune than filters having multiple cross couplings.
Conventional filters of the type just described suffer from significant drawbacks. Specifically, filter 10 requires three cross couplings. Filters with multiple cross couplings are difficult to tune and are not appropriate for some applications. Filter 27 requires the use of non-adjacent element coupling and is not symmetrical.